The invention relates to a method for the operation of a fuel cell battery comprising a control system. It also relates to a plant with a fuel cell battery, the operation of which is controlled in accordance with the method in accordance with the invention.
Fuel cells, in particular high temperature fuel cells of the SOFC type (xe2x80x9cSolid Oxide Fuel Cellxe2x80x9d), enable a utilization of energy of a fuel through an energy conversion. In this, both electrical energy, which is produced as a result of electrochemical processes, and thermal energy, which arises in the form of hot exhaust gases, can be used. Gaseous flows of two educts are passed through the cells separately. The first educt, which is in particular ambient air, contains oxidizing components, the second educt contains reducing components. A gas containing methane (e.g. natural gas) is advantageously used as the second educt, and is passed prior to entry into the cells through a reformer and is for example converted there into the reducing components hydrogen and carbon monoxide by means of an additional infeed of water and with process heat being supplied. The hot exhaust gas can advantageously be used as the source for the process heat which is required in the reformer. In the following description no distinction will be made between the flows of an original educt and those of a converted educt.
The reforming processes will as a rule no longer be explicitly named; rather it will be tacitly assumed that the use of reformers is also provided for.
The educts are fed into the battery in a controlled manner, and indeed in a conditionally predetermined ratio of quantities, for which a safety condition must be fulfilled: No reducing components, that is, no unburned components of the second educt, may any longer be contained in the exhaust gas flow. Thus a gas safety system must be provided, by means of which a situation is achieved in which xcexxe2x89xa71 for the air index (or air ratio) xcex which is related to the stoichiometric relationship. In fan driven burners, pneumatic mixing devices are used which enable a variable supply of the educt flows into a combustion chamber with xcex remaining constant. Such devices operate with a balanced pressure regulator or a pressure ratio regulator. During the development of the control system for plants with fuel cells, the idea arose of using the mixing apparatuses which are known from fan driven burners in a suitably modified form in the control system to be developed.
The electrical energy requirement of the consumer is not constant as a rule, so that the load which is placed on the fuel cell battery correspondingly has a fluctuating variation. Load variations of this kind result in temperature fluctuations in the cell. The reaction temperature must be kept by means of a regulation to within an interval in which the temperature is sufficiently high for efficient processes and is sufficiently low with respect to the materials used.
The educt flows are advantageously driven by sucking away the exhaust gas by means of a fan of which the speed of rotation is controlled. The reaction temperature can be regulated by means of variations of the speed of rotation of the blower. If the named pneumatic device is to be used, then a problem arises which is not known from the fan driven burners: If the consumer for example draws less electrical energy, then with the educt flows remaining unchanged the production of waste heat increases correspondingly. If now the air flowxe2x80x94the flow of the first eductxe2x80x94is intensified in order to be able to remove surplus heat from the cells to an increased extent, then the supply of the second educt also increases. This would however result in an intensified turnover of the electrochemical reaction in contradiction to the reduced demand for electrical energy.
The object of the invention is to create a method by means of which the said pneumatic device with pressure regulator can be usedxe2x80x94suitably modifiedxe2x80x94in a plant with a fuel cell battery and in which a temperature regulation with variable flow of the first educt (as a rule, ambient air) is possible. This object is satisfied by the method which is defined in claim 1.
The method for the operation of a fuel cell battery comprises a control system, through which the electrochemical reactions in cells of the battery are influenced. Gaseous flows of two educts are fed into the battery in a controlled manner in a conditionally predetermined ratio of quantities and are passed through the cells separately. The first educt contains oxidizing components, the second educt containing reducing components and the first educt is in particular ambient air. The educt flows are united after passage through the cells and are further treated by means of an afterburning process and with the production of a flow of exhaust gas, so that, at the conditionally predetermined ratio of quantities, the reducing components are completely oxidized. The first educt flow, in particular the air flow, can be varied by the control system to a limited extent; it is used for a regulation of the reaction temperature. Simultaneously to the regulation of the reaction temperature the second educt flow is held constant, namely by means of a second regulation, which acts on one of the educt flows upstream of the battery inlet.
The subordinate claims 2 to 9 relate to advantageous embodiments of the method in accordance with the invention. A plant with fuel cells which is operated in accordance with this method is the subject of claim 10.
The quality of the fuel can vary, so that fixed values for the proportions of the reducing components can not be assigned to the mass flow of the second educt in a unique way. The method in accordance with the invention can be further developed in such a manner that fluctuations in the fuel quality can also be taken into account. For this for example a xcex-probe or a CO sensor is employed, which is used for measuring the residual proportion of oxygen or the residual carbon monoxide CO in the exhaust gas flow respectively. Changes of the remaining proportion of oxygen and/or of the remaining carbon monoxide CO can be considered as a measure for the changes of quality. The regulation of the second educt flow, as is provided in the method in accordance with the invention, can then be extended such that regulation is carried out to hold constant the residual proportion of oxygen in the exhaust gas flow.
The quality of the second educt can also be characterizable by another parameter, for example by the proportion of hydrogen. A sensor for measuring this characteristic parameter can be arranged in the second educt flow, so that a variation in the characteristic parameter can be used to influence of the second regulation process. With this, the carrying out of the second regulation is likewise possible as a result of a change of quality of the second educt.